In-plane electric fields are used for the alignment of liquid crystals and other molecules, or for the movement of charged particles. In-plane switching of liquid crystals is used in displays, as well as in other optical applications. Some electrophoretic display architectures use in-plane electric fields to move colored particles for reflective displays. Another application of in-plane electric fields is for dust shields which are used for removal of dust or prevention of dust build-up on surfaces. Dust shields are particularly useful for cleaning windows, or surfaces in solar power systems such as photovoltaic (PV) surfaces (e.g., silicon solar panels), or mirrors for use in concentrated solar power (CSP).
U.S. Pat. No. 6,911,593 to Mazumder et al., entitled “Transparent self-cleaning dust shield,” describes a transparent self-cleaning dust shield, also referred to as an electrodynamic shield (EDS), that has electrodes embedded within a thin transparent dielectric film or a sheet and is used to remove dust deposited on solar panels. The electrodes are in the same plane, and are therefore “co-planar.”
Some of the major difficulties in applying EDS on solar panels include: (1) avoiding interactions between the electric field of the EDS electrodes and current collecting grids used in solar panels for providing electrical power; (2) scaling of the method of EDS construction for manufacturing and installing transparent electrodes on solar panels and solar concentrators; (3) obscuration of solar radiation caused by the placement of the EDS on the surface of solar panels and concentrators; (4) retrofitting existing solar photovoltaic and photothermal devices with self-cleaning EDS systems; (5) environmental degradation of polymer films under outdoors condition; (6) maintaining the efficiency of heat dissipation of solar panels integrated with EDS; and (7) cost-effective manufacturing of new solar panels and solar concentrators integrated with EDS for large-scale installations.
To address issue (2), new electrode geometries are needed which are robust, and avoid the need for multiple patterning steps. Current fabrication methods utilize patterned dielectric materials between electrode or bus cross-over points. This patterned dielectric adds manufacturing complexity due to additional process and pattern-alignment steps. Additionally, the patterned dielectric materials typically suffer from robustness issues, with defects or inherent materials properties leading to dielectric breakdown and electrode shorting.
Both displays and dust shields benefit from the use of transparent electrodes to generate the in-plane electric field. Transparent conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal displays. Transparent conductive electrodes are also used in touch-screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 70%, and more preferably greater than 90%, in the visible spectrum) and a high conductivity (for example, less than 10 ohms/square).
Typical prior-art conductive electrode materials include indium tin oxide (ITO), and very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example, by sputtering or vapor deposition, and patterned on display or touch-screen substrates, such as glass. However, the current-carrying capacity of such electrodes is limited, thereby limiting the length of electrode due to the resultant resistive losses. Moreover, the substrate materials are limited by the electrode material deposition process (e.g., sputtering). Thicker layers of metal oxides or metals can increase conductivity but reduce the transparency of the electrodes.
It is also known in the prior art to form conductive traces using nano-particles including, for example, silver. The synthesis of such metallic nano-crystals is known. For example, issued U.S. Pat. No. 6,645,444 to Goldstein, entitled “Metal nano-crystals and synthesis thereof,” describes a process for forming metal nano-crystals optionally doped or alloyed with other metals. U.S. Patent Application Publication 2006/0057502 to Okada et al., entitled “Method of forming a conductive wiring pattern by laser irradiation and a conductive wiring pattern,” describes fine wirings made by drying a coated metal dispersion colloid into a metal-suspension film on a substrate, pattern-wise irradiating the metal-suspension film with a laser beam to aggregate metal nano-particles into larger conductive grains, removing non-irradiated metal nano-particles, and forming metallic wiring patterns from the conductive grains. However, such wires are not transparent and thus the number and size of the wires limits the substrate transparency as the overall conductivity of the wires increases.
There is a conflict between the optical transparency and the conductivity (or surface resistance) of transparent electrodes. For example, copper films having a surface resistance of about 0.25 milliohms/square are commercially available, but their optical transparency is well below the desired level of 70%. Other commercially available thin-films formed from conductive materials such as ITO or silver have acceptable transparencies (for example, AgHT™ silver type films have optical transparencies greater than 75%), but such films have surface resistances in the range of 4-8 ohms/square, which is several orders of magnitude greater than that of the above copper films, or conventional conductors used for antenna construction.
There remains a need for robust electrode films for in-plane electric field generation that are capable of delivering the required high electric field strength over large areas, that are easy to connect with external power sources, are free from issues related to shorting, and are additionally easy to manufacture. There is a further need for these electrode films to be highly-transparent to minimize their impact on solar efficiency, in the case of EDS films for PV or CSP, or to minimize their visual-impact, in the case of displays or other direct-view in-plane field devices such as smart windows.